1. Field of the Invention
The present invention relates to a linear motion rolling guide unit which is applied to sliding portions of machining tools and which consists of a track rail with raceway grooves formed longitudinally extending on both side walls thereof, a casing saddling the track rail and having raceway grooves, and a number of rolling elements that roll between the facing raceway grooves.
2. Description of the Prior Art
In a conventional linear motion rolling guide unit, a slider that slides on a track rail consists of a casing, end caps fixed to both ends of the casing, end seals attached to the end caps, and an under seal provided to the undersides of the casing and the end caps. The linear motion rolling guide unit further consists of: a number of rolling elements that travel rolling in a raceway formed between the raceway surface on the track rail and the raceway surface on the casing; and a retainer fixed to the casing to hold the rolling elements in the casing.
The linear motion rolling guide unit as described above will be explained by referring to FIGS. 5, 6 and 7. FIG. 5 is a perspective view showing one example of a conventional linear motion rolling guide unit. FIG. 6 is a cross section of FIG. 5 taken along the line A--A, and FIG. 7 is a cross section taken along the line B--B of FIG. 6. The linear motion rolling guide unit shown in FIG. 5 is a four-raceway endless linear motion rolling guide unit. It has a track rail 1 with an I-shaped cross section, a slider 2 straddling the track rail 1 in such a way that it can move relative to or slide on the track rail 1, and a number of cylindrical rolls 5 rotatably interposed between the track rail 1 and the slider 2. The track rail 1 is formed with recessed grooves 8 longitudinally extending on both sides thereof, which form raceway surfaces 9. A recessed portion 11 of the casing 4 is formed with a raceway surface 10 (FIG. 7) at a position facing the raceway surface 9 on the track rail 1.
The slider 2 has a casing 4 which is formed with the recessed portion 11 so that the casing can saddle the track rail 1, and end caps 6 attached to both longitudinal ends of the casing 4. To seal the boundary between the track rail 1 and the slider 2 when the slider 2 slides on the track rail 1, the ends of the end caps 6 are each provided with an end seal (not shown) and the underside of the slider 2 with an under seal 25C.
Between the upper and lower raceway surfaces 10 of the casing 4 is formed an engagement groove 15, which engages with an engagement projection 16 formed on the retainer 3. A part of the retainer 3 is loosely fitted in the recessed groove 8 of the track rail 1 and fixed to the casing 4 by bolts 17.
In the above construction of the 4-raceway endless linear motion rolling guide unit, two raceways are formed on each side of the track rail by the upper and lower raceway surfaces 9 on the track rail 1 and the upper and lower raceway surfaces 10 on the casing 4. Hence, a total of four raceways are formed on both sides of the track rail 1. In these loaded roller raceways, a number of cylindrical rollers 5 roll in contact with the facing raceway surfaces 9 and 10. The casing 4 is formed with return passages 13, 14, and the end caps 6 are formed with direction changing passages 12 that connect the loaded roller raceways and the return passages 13, 14. To form the direction changing passage 12, the end cap 6 is formed on its inner side with an recessed arc surface 19, as shown in FIG. 7. A direction changing passage spacer 7 is disposed facing, and spaced from, the recessed arc surface 19 to define the direction changing passage 12. FIG. 7 shows only one of the endless circulating passage.
With the 4-raceway endless linear motion rolling guide unit, the loaded roller raceways, the direction changing passages 12 and the return passages 13, 14 combine to form two endless circulating passages. The endless circulating passages may, for example, have different lengths and be disposed one inside the other. That is, a smaller endless circulating passage may be arranged inside the loop of a larger endless circulating passage and disposed at right angles with each other so that they will not interfere with each other. The larger and smaller endless passages have the same length of the loaded roller raceways. As the slider 2 moves along the track rail 1, the cylindrical rollers 5 circulate through the large and small endless circulating passages. It is also possible to make the endless circulating passages cross each other so that they can be formed in the same length.
Such a four-raceway endless linear motion rolling guide unit is cited, for example, in the Japanese Patent Laid-Open No. 175564/1989, Japanese Utility Model Laid-Open No. 63812/1992, and Japanese Patent application No. 166326/1991.
In the conventional four-raceway endless linear motion rolling guide unit, the direction changing passage 12 is defined by interposing the spacer 7 between the casing 4 and the recessed arc surface 19 when the end caps 6 are attached to the casing 4. The direction changing passage 12 is supplied with grease as lubricant to make the rolling of the cylindrical rollers 5 smooth. However, a problem may arise that as a large number of rollers 5 travel through the direction changing passage 12, the grease sealed into the passage 12 may cause large rolling friction, i.e., sliding resistance between the rollers 5 and the spacer 7.